Polymerisable mesogenic or liquid crystal (LC) compounds, which are also known as reactive mesogens (RM), have been described in prior art for various purposes. For example, they can be used for the preparation of linear or crosslinked liquid crystal side chain polymers. Furthermore, they can be aligned in their liquid crystal phase and subsequently polymerized in situ to give linear or crosslinked liquid crystal polymer films with uniform orientation of high quality. These films can be used as optical elements like polarisers or compensators in flat panel displays, as described for example in EP 0 397 263, WO 98/00475, WO 98/04651 or WO 98/12584.
RMs have also been suggested for use in polymerised cholesteric liquid crystal films or coatings that show selective reflection of visible light and are suitable as optical films like narrowband or broadband reflective polarizers or colour filters as described for example in EP 0 606 940 or WO 97/35219, or for the preparation of liquid crystal pigments, as described for example in WO 97/30136. Other important fields of use are security markings as described for example in U.S. Pat. No. 5,678,863 or hot stamping foils as described for example in GB 2,357,061.
Furthermore, isomerisable RMs are known in prior art which show E-Z or cis-trans isomerisation upon photoirradiation and are thereby change their shape and physical properties like the birefringence or, in case of chiral compounds, their chirality and twisting power. Such photoisomerisable RMs have been suggested for example for the preparation of cholesteric polymer films with patterned optical properties, which can be used as optical components like colour filters or broadband reflective polarizers in liquid crystal displays. The preparation of patterned cholesteric films is described for example in WO 00/34808. Also, they can be used as photoorientable materials for applications using photoalignment by irradiation with linear polarized light, like the preparation of alignment layers or optical films having a pattern of regions with different orientation, as described for example in U.S. Pat. No. 5,602,661. Furthermore, photoisomerisable RMs have been suggested for use in cholesteric or multi-domain liquid crystal displays, as disclosed in WO 98/57223.
It is known that RMs comprising a cinnamic acid group are suitable as photoisomerisable or photoorientable materials. Such compounds are described for example in U.S. Pat. No. 5,770,107.
However, the compounds of prior art are often difficult to synthesise. The intermediate cinnamic acids show only limited solubility in organic solvents which is disadvantageous for synthesis. For example, the prior art materials cannot be reacted under standard Dean and Stark conditions because the material is insoluble in dichloromethane DCM. This creates problems because quantities of THF has to be added in order to aid solubility of the intermediate cinnamic acid to allow reaction to occur. This process is far from optimal, because even in THF the intermediate is only partially soluble. For example 20 volumes THF can be used to recrystallise the cinnamic acid intermediate. In contrast the new, laterally siubstituted cinnamic acids (used to synthesise materials of Formula I) are soluble in DCM, enabling them to be reacted using standard Dean & Stark conditions.
Therefore, there is a demand for photoisomerisable or photo-orientable RMs which are easy to synthesize in a large range of derivatives, show good solubility in DCM, have a low melting point, do not negatively affect the liquid crystal phase behaviour of LC mixtures or polymerisable LC mixtures and are suitable for the uses as mentioned above. The materials should preferably posses a broad liquid crystal phases, in particular a nematic phase, and be highly miscible with other reactive liquid crystal materials. In this way they can posses suitable values of the birefringence Δn ranging preferably from 0 to 0.4.
The invention has the aim of providing photoisomerisable and photoorientable polymerisable mesogenic or liquid crystal compounds having these properties, but not having the disadvantages of the compounds of prior art as discussed above. Another aim of the invention is to extend the pool of reactive photoisomerisable compounds available to the expert.
The inventors of the present invention have found that these aims can be achieved by providing compounds as claimed in the present invention.
Definitions of Terms
The term ‘film’ as used in this application includes self-supporting, i.e. free-standing, films that show more or less pronounced mechanical stability and flexibility, as well as coatings or layers on a supporting substrate or between two substrates.
The term ‘mesogenic compounds’ as used in the foregoing and the following should denote compounds with a rod-shaped, lath-shaped or disk-shaped mesogenic group, i.e. a group with the ability to induce mesophase behaviour. These compounds do not necessarily have to exhibit mesophase behaviour by themselves. It is also possible that these compounds show mesophase behaviour only in mixtures with other compounds or when the mesogenic compounds or the mixtures comprising them are polymerized. Rod-shaped and lath-shaped mesogenic groups are especially preferred.
For the sake of simplicity, the term ‘liquid crystal (LC) material’ is used hereinafter for both liquid crystal materials and mesogenic materials, and the term ‘mesogen’ is used for the mesogenic groups of the material.
Polymerisable compounds with one polymerisable group are also referred to as ‘monoreactive’ compounds, compounds with two polymerisable groups as ‘direactive’ compounds, and compounds with more than two polymerisable groups as ‘multireactive’ compounds. Compounds without a polymerisable group are also referred to as ‘non-reactive’ compounds.
A polymerisable mesogenic or liquid crystal compound is also shortly referred to as ‘reactive mesogen (RM)’.